1. Field of the Invention
The present invention relates to optical components. The present invention relates to an optical component that shields visible light, particularly used in the infrared optical system of an infrared application apparatus such as an infrared sensor, an infrared thermal image processor, and an infrared laser, a zinc sulfide sintered compact, and a method of fabricating the same.
2. Description of the Related Art
The development of a new and highly functional device taking advantage of the superior feature of the infrared ray is now in progress. As to the applications adapting the sensing function, security systems utilizing body sensors, surface thermometers measuring the surface temperature of an object in a non-contact manner, resource probe systems identifying the resource distribution of earth from high above, devices detecting an object in the dark field, and gas analysis devices can be enumerated. Also, infrared thermal image processors for processing collected data by the above devices, high power laser processors utilizing the heat energy of infrared rays are known.
In accordance with the development of such new high functional infrared application apparatuses, the demands placed on higher practical features and lower cost have become critical for the components directed to optical functions used in respective apparatuses such as the window member, the lens member and the like.
As to these optical components, the conventional single crystal germanium (Ge), polycrystals of zinc selenide (ZnSe) and zinc sulfide (ZnS) by chemical vapor deposition (CVD), and infrared transmissive glass including arsenic (As) and serene (Se) are known as the optical materials accommodating the wavelength range of 8-12 .mu.m. Development for practical use is now in progress on the basis of the superior infrared transmitting properties.
Ge is extremely expensive due to its limitation in resource. ZnS and ZnSe produced by CVD have problems with respect to the environment and production since toxic gas is used during the production stage and that the deposition rate from vapor is low. It is difficult to reduce the cost thereof. Furthermore, infrared transmissive glass includes toxic components such as As and Se, imposing counter-environmental problems. Therefore, the aforementioned materials are currently used only in limited applications such as for military usage, small optical components, carbon dioxide gas lasers and the like.
There has been intensive efforts to find and produce a material having high transmittance in the wide infrared region taking into account the issues of counter-environment and productivity. Particularly, ZnS has attracted a lot of research efforts since it does not include any toxic elements. Research and development of a sintered compact (polycrystal) along this line is in progress by means of hot pressing, eliminating the usage of toxic gas as the material in the production stage.
Japanese Patent Publication No. 41-412 discloses a method of fabricating a polycrystalline ZnS sintered compact having the theoretical density of 99-100% by means of hot pressing ZnS powder in vacuum or inert gas under the condition of 1.4-2.9 ton/cm.sup.2 in pressure and 770-965.degree. C. in temperature. This publication discloses that a sintered compact of various shapes such as in the form of a dome, a lens and the like is obtained by this method. It is noted that the transmittance of a sample of 1.6 mm in thickness thereof exhibited a high level value exceeding 60% in the wide infrared region of 2-6 .mu.m.
Japanese Patent Laying-Open No. 50-2006 discloses a method of obtaining a light transmissive ZnS polycrystal by setting a forming piece of only ZnS powder or of mixture powder thereof with alkali metal halide in a mold and applying a hot press process under the condition of 50-500 kg/cm.sup.2 in pressure and 600-1500.degree. C. in temperature for at least 5 minutes. Example 1 thereof discloses the steps of placing a ZnS powder forming piece in a graphite pressurizer mold, hot pressing the same under the condition of not more than 10.sup.-3 Torr in vacuum, 0.2 ton/cm.sup.2 in pressure, and 1000.degree. C. in temperature for 30 minutes, and polishing the plate to 50 mm in diameter and 3 mm in thickness. The light transmittance up to the wavelength of 2.5 .mu.m of this plate is disclosed in FIG. 1 of this publication Japanese Patent Laying-Open No. 50-2006. It is appreciated from this FIG. 1 that the transmittance is 4-18% at the visible light region (wavelength 0.4-0.8 .mu.m) and 19% at the wavelength of 2.5 .mu.m in the near infrared region. It is therefore considered that a sample of 2 mm in thickness can exhibit higher light transmittance.
In pp. 2086-2092 of "Journal of the American Ceramic Society" Vol. 76, No. 8, a ZnS polycrystal is introduced obtained by using ZnS material powder having the grain size distribution width of 2-4 .mu.m with the average grain size of 2 .mu.m and 99.99% in purity and applying to the powder various uniaxial pressures (in Table 1 of this document, 137-207 MPa, i.e., 1.4-2.1 ton/cm.sup.2) under vacuum of approximately 5.times.10.sup.-2 Torr at 950.degree. C. for 40-50 minutes in a uniaxial hot press apparatus of the graphite heater system. Eventually, a solidified ZnS polycrystal disc of 12.7 mm in diameter has the density of approximately 99.6-99.8% by the X-ray theoretical density according to Table 1 of the document. The infrared transmittance is approximately 40-70% for the infrared ray of 2.5-3 .mu.m in wavelength according to FIG. 1 of that document.
FIG. 3 of the same document discloses the calculated values of the infrared transmittance for a sample disc of 2 mm in thickness under the assumption of various porosity levels from 0.01% to 1% with the pore of 0.3 .mu.m in diameter. According to FIG. 3, light transmittance of the sample disc with the porosity of 0.5% is 0% for 2.5-3 .mu.m in wavelength and approximately 40-60% for 8-10 .mu.m wavelength. Under the assumption of 0.05% for the porosity, the light transmittance is approximately 15-25% at the wavelength of 2.5-3 .mu.m and approximately 70% at the wavelength of 8-10 .mu.m. This document discloses that the porosity must be less than 0.01% in order to use this type of polycrystalline zinc sulfide for a through-window of infrared rays from the calculated values. It is also mentioned that such a polycrystal of the same porosity level cannot be easily produced by the general sintering or hot press method.
Japanese Patent Publication No. 1-55213 discloses a polycrystalline ZnS sintered compact having the transmittance of at least 30% in the area of 3 mm in thickness at the infrared region of 1-14 .mu.m in wavelength by hot pressing ZnS powder of high impurity with the grain size of not more than 5 .mu.m in vacuum with the pressure of 0.8-1.4 ton/cm.sup.2 and the temperature of 800-1050.degree. C. Typical transmittance values of a polycrystalline ZnS sintered compact disclosed in this publication are shown in FIGS. 1 and 2. The sample of FIG. 1 has superior transmittance at the wavelength range of 8-12 .mu.m. It is also appreciated that the sample of FIG. 2 is superior of transmittance at the wavelength range of 2.5-3 .mu.m than that of FIG. 1.
The infrared sensor technology used in combination with the above conventional infrared optical components has seen significant progress these few years. The conventional infrared sensor uses the HgCdTe type material oriented to the wavelength range of 10 .mu.m. It was necessary to cool down such a conventional infrared sensor to an operable low temperature using liquid nitrogen and the like. Recently, uncooled type infrared sensors have emerged adapting bolometer type detector, pyroelectric type detector, thermocouple type detector and the like.
These uncooled type infrared sensors have sensitivity for a wider wavelength range than that of the conventional cooled type sensor. For example, it is sensitive to rays of a wider wavelength range from visible light to infrared rays. Therefore, the uncooled type infrared sensor will sense and react to infrared rays of less than 5 .mu.m in wavelength, to near-infrared rays of not more than 3 .mu.m in wavelength, and also to visible light of 0.4-0.8 .mu.m in wavelength, in addition to the infrared rays of 8-12 .mu.m that is essentially required for body sensing. This sensitivity induces the problem of erroneous operation and sense precision error.
It is necessary to cut off light of the short wavelength region that is the cause of noise, particularly visible light, to solve this problem. This was impossible with the conventional optical component that exhibits constant transmittance at the aforementioned wide wavelength region. Therefore, measures such as providing a filter, for example, that cuts off the visible light have been taken. Since this will increase the cost, it is desirable to apply selective light transmittance to the optical component material per se serving as the window member and the like.